401(k) investors can thank just 21 superstar stocks for driving the big gains in their retirement savings accounts this year.

But is that a good thing?

The so-called 21 Club, which includes U.S. tech stocks like Apple, Facebook and Amazon and blue chips like Visa, Home Depot and McDonald's, has accounted for more than half (50.7%) of the Standard & Poor's 500 stock index's 17.2% total return this year, according to S&P Dow Jones Indices data through Nov. 14. Total return includes the increase in a stock's price and its dividend payouts.

The S&P 500, an index of the nation's 500 biggest companies, is often dubbed "the market."

But a market driven by just a handful of superstars — Apple alone accounts for nearly 10% of the index's total return this year — is not always viewed positively by Wall Street.

Analysts prefer to see the market propelled higher by a larger group of stocks. Oftentimes, a "narrow" market can mask weakness and, like in 1999-2000 when it was just tech stocks leading the march higher near the end of the bull run, it can signal a peak and the danger of a big drop.

"The past has shown that there are risks when returns are concentrated in so few stocks," says Mike Venuto, chief investment officer and co-founder of New York-based Toroso Investments, an ETF research firm. "People think they are diversified, but the reality is that their money is concentrated in less than two dozen stocks."

The "Nifty Fifty" stocks of the 1960s and 1970s eventually got creamed in the bear market of the mid-70s, just as the dot-com darlings of 1999 suffered massive losses in the 2000-2002 bear market.

More recently, the FAANG stocks — Facebook, Apple, Amazon, Netflix and Google parent Alphabet — have led the performance charts, notes Jamie Cox, managing partner at Harris Financial in Richmond, Va. Tech stocks have accounted for nearly 45% of the S&P 500's total return this year, according to S&P Dow Jones Indices.

So what kind of market do we have now? A winner-take-all that will continue to benefit investors who put their money in the S&P 500 through index mutual funds or ETFs? Or a market that's showing signs of fatigue with too few stocks pulling their weight?

That's a key question given that Main Street investors had $806 billion riding on S&P 500 index mutual funds at the end of 2016, more than double the amount at the end of 2007, according to the Investment Company Institute, a fund industry trade group. The S&P 500 is weighted by the market value of its stocks, which means the bigger companies have an outsized impact on the index's price moves.

Over the past 10 years, investors have been fleeing domestic stock funds run by money managers who do their own research and pick their own stocks. Instead, they are flocking to low-cost ETFs and index funds that track or mimic stock indexes like the S&P 500. From 2007 through 2016, $1.4 trillion in net new cash flowed into U.S. index funds and ETFs, vs. outflows of $1.1 trillion for funds run by portfolio managers, ICI data show.

That shift in investment strategy means many Main Street investor portfolios now rise and fall with the most popular stocks of the day.

And no investment is risk free, especially one with such broad popularity.

Still, there is a debate on Wall Street now as to how concentrated the market really is at the moment.

One camp, which includes Erik Davidson, chief investment officer at Wells Fargo Private Bank, is wary of the recent trend towards fewer stocks leading the advance.

"Twenty-one stocks accounting for 50% of the market’s return seems excessive," he says, adding that there's risk if those current market leaders falter and the funds that track them go down in value. It would be more reasonable, he says, to have 50 to 100 stocks in the S&P 500 accounting for half of its return.

"Investors need to know that risk," he says. "At some point, we could shift into a 'bigger they are, the harder they fall' market."

Adds Cox: "If the market leaders drop dramatically you will have a disproportionate impact on the down side. It works both ways."

But not everyone agrees — at least not yet.

The 10 biggest stocks in the S&P 500 now account for roughly 20% of the index, well below the peak of nearly 27% back in 2000, says Todd Sohn, analyst at Strategas Research Partners, an investment firm in New York. "The reading in 2000 was much more evident of a narrowing market," he says.

What's more, he stresses that the market is simply separating winners from losers more than they did in the early years of the bull market, which began in 2009. He said that 38% of S&P 500 companies are up 20% or more this year, while 29% have fallen.

Nearly nine years into the bull market, the statistics merely show that "all stocks aren't moving in one direction together," says Lindsey Bell, investment strategist at CFRA Research in New York. "It shows the market is working more normally. Investors are rewarding companies with strong fundamentals and outlooks and (punishing) companies with deteriorating outlooks."

Ryan Detrick says concentration at the top in today's market is not nearly as dangerous as it was in 2000. The list of stocks, he notes, aren't all tech companies.

"It doesn't feel as one-sided or lopsided as we saw in the tech bubble in the 90s," he says.